Near-Infrared Photochemistry of Atmospheric Nitrites

Paul Wennberg, Coleen Roehl, Geoff Blake, and Sergey Nizkorodov

California Institute of Technology

Ross Salawitch, Geoff ToonJet Propulsion Laboratory

Stratospheric Ozone Photochemistry

Courtesy NASA Goddard

HO2 + O3 OH + O2 + O2

OH + O3 HO2 + O2

O3 + O3 O2 + O2 + O2

Catalytic destruction of Ozone by HOx

Wennberg et al., Science, 266, 398, 1997

HOx PhotochemistrySources:

O3 + hν (< 314 nm) O (1D) + O2 O (1D) + H2O 2 OH

Sinks (Direct):OH + HO2 H2O + O2

Sinks (Indirect):OH + NO2 HONO2

OH + HONO2 H2O + O3

HO2 + NO2 HO2NO2

OH + HOONO2 H2O + O2 + NO2

OH HO2

NO, O3

O 3

Tro

posp

heri

c O

3 Pr

oduc

tion OH + CO CO2 + HO2

HO2 + NO NO2 + OH NO2 + hν (< 450 nm) NO + O

O + O2 O3 Net: CO + 2 O2 O3 + CO2

More O 3 production

Less O3 production

Jaegle et al., J. Geophys. Res., 105, 3877-3892, 2000.

O2

The Color of Sunlight

Peroxy Nitric Acid (HO2NO2)Donaldson et al. (1997) proposed that dissociative excitation of OH vibrational overtones in H2O2, HNO3, and HO2NO2 is an additional source of OH in the atmosphere

Wennberg et al. (1999) found unknown photochemical source of OH in the mid-latitude stratosphere with photolysis > 650 nm and suggested HO2NO2 as the carrier

D. J. Donaldson et al., Geophys. Res. Lett. 24, 2651 (1997)

P. O. Wennberg et al., Geophys. Res. Lett. 26, 1373 (1999)

Solar Flux

Wavelength [nm]1000

Sola

r Fl

ux [P

hoto

ns c

m-2

s-1 n

m-1

]

1012

1013

1014

1015

SZA = 0ºSZA = 86º

300 400 500 700

Near IR solar flux is orders of magnitude higher than UV flux

Approach

HO2NO2 + h HO2 + NO2

HO2 + NO OHOH + NO2

1. IR-photodissociation2. Conversion into OH3. Detection of OH

Vibrational Dissociation Spectroscopy

Experiment

• Direct overtone pumping of CH / OH stretches in PAN / PNA / HOONO• Chemical conversion of photodecomposition products into OH radicals• LIF detection of OH in a single photon counting regime

Wavenumber [cm-1]6000 7000 8000 9000 10000

31

21

21+3

1+23

Sample action and FTIR spectra of PNA

D0

Action Spectra

• Different relative band intensities in FTIR and action spectra • Dissociation quantum yields determined by comparing spectra• Initial internal energy responsible for dissociation below D0

absdissPNAaction σφρ S

absPNAFTIR σρ S

(21, 240 K)=14%

Phot

odis

soci

atio

n C

ross

Sec

tion

(cm

2 mol

ecul

e-1 c

m-1

)

10-20

10-19

10-18

Qua

ntum

Yie

ld

0.1

1

Dissociation Cross-Sections and Quantum Yields

10-21

10-20

10-19

0.1

1

1000/T (K-1)3.4 3.6 3.8 4.0 4.2 4.4

10-22

10-21

10-20

0.01

0.1

2+

2

+2

Temperature-Dependent Action Spectra of PNA

Wavenumber [cm-1]650067507000725097501000010250

Inte

nsity

[a.u

.]

31

224 K

244 K

283 K

21

• Relative band intensities in action spectra of PNA are T-dependent

• {diss(31) = 1} Absolute photodissociation cross sections and quantum yields for other bands

MkIV HO2NO2 Observations

Frequency (cm-1)

Inte

nsity

R

esid

ual (

%)

OH + NO2 ??• The Reaction of OH with NO2 is among the

most important reactions in Earth’s atmosphere. By sequestering both HOx and NOx it essentially shuts down reactive photochemistry.

• It is assumed by all models that the only product formed is nitric acid

Part II. HOONO

• Suspected intermediate of the OH + NO2 association reaction

• Proposed intermediate of liquid phase reactions of peroxynitrite ion (ONOO-)

• Observed in rare-gas matrices in 1991 Cheng et al. J. Phys. Chem. 95, 2814 (1991)

• Extensively studied by theory– At least three stable conformers– Bound by 19 kcal/mol

• Never observed in the gas phase

HOONO Atmospheric Significance Reaction Intermediates

HO + NO2

HOONO-19

(3 isomers)

HONO2

+7

-48

0

HO2 + NO

HONO2 HOONOIrreversible Reversible

Removal of HOx and NOx

No effect

Produce HOONOHOONO directly in the gas-phase

H2 + μwave discharge 2 H

H + NO2 OH + NO

OH + NO2 + M HNO3 + M

OH + NO2 + M HOONO OONO + M

Photofragment:

HOONO OONO + hν OH + NO2

Detect OH by LIF

Preparation

Observed Spectra

Stronger peaks assigned to HOONO 21 overtones and combination bands

Assignment for weaker bands remains ambiguous

Intensities affected by photodissociation dynamicsD0

21

Observed HOONO Yield

• HOONO lifetime unknown lower limit• Different conditions incomparable• Higher yield expected for upper troposphere

This work(253 K, 20 Torr)

Burkholder et al.

Dransfield et al.

Hippler et al.

> 5 3 % (after 300 ms)

< 5 % < 10 % Unpublished

Future Projects• Photochemistry of reaction intermediates

– HOCO– HOOOCl– CH3OONO

• Chemistry and kinetics of weakly-bound molecules– CH3OONO2

– CH3C(O)OONO2– HOONO– HO2NO2

• UV photodissociation spectroscopy of atmospheric molecules– CH3OOH– HO2NO2

Thanks

• Funding by NASA and NSF• Support for Sergey Nizkorodov (just

appointed assistant professor of chemistry UC-Irvine) from the Dreyfus foundation.

• You for your attention!